I agree that a proof-of-work based consensus algorithm (like in bitcoin, and in my P2P exchange proposal) is energetically much more expensive than the Ripple consensus algorithm (which is described at https://ripple.com/wiki/Consensus ).

And I agree that there's no reason why a P2P exchange has to use a proof-of-work based consensus algorithm - it just needs to use a well-proven P2P consensus algorithm.

But ... the bitcoin consensus algorithm has been demonstrated to work well over the last four years.

How confident are people in the Ripple consensus algorithm (which hasn't been tested yet)?

What other consensus algorithms can be considered well-proven in the face of huge botnets and hostile clients?